Ataxia-telangiectasia (A-T) is a progressive genetic disease characterized by cerebellar degeneration, relentless motor dysfunction, telangiectasias, immunodeficiency, chromosomal instability, sensitivity to ionizing radiation and radiomimetic chemicals, and a striking predisposition to cancer. A-T carriers show mild radiation sensitivity and an excess of malignant diseases. The disease involves at least four different genes, whose protein products are as yet unknown. Two of the A-T genes, ATA and ATC, together accounting for more than 80% of the patients, are closely linked at the chromosomal region 11q22.3-23.l, within an interval of 3-4 megabases. The aim of the proposed project is to identify and clone the ATA and ATC genes. Towards this goal we are employing an experimental approach that combines two different strategies for the isolation of disease genes with unknown protein products: l. Positional cloning: genetic analysis is being used to narrow the genomic interval to which the two genes have been mapped and the region is being cloned in yeast artificial chromosomes and cosmids. Transcribed sequences will be sought within this interval, and the A-T genes will be identified among them using a functional assay (complementation of the cellular phenotype) and by screening for defects in patients. 2. Complementation cloning: an attempt is being made to "correct" the hypersensitivity of A-T cell lines to radiomimetic chemicals by introducing cDNA libraries made in episomal expression vectors, and identifying cDNA clones that increase the cells' resistance to the drugs. Such cDNAs will be mapped to the ATA/ATC genomic contig and further tested by searching for mutations in patients. These two different approaches are thus interactive and converge at various stages. Isolation of the ATA and ATC genes will lead to identification of their protein products and the molecular defects in patients, to better understanding of the role of these genes in the development of the nervous and immune systems, elucidate their involvement in genetic predisposition to cancer, and result in the development of molecular probes for prenatal diagnosis and carrier detection in A-T.